A large number of devices are operated with a drive signal. Such devices may include among others haptic actuators, speakers and crystal oscillators. Haptic actuators are commonly used with electronic devices to provide the user with a sensory signal also referred to as haptic feedback. For example, mobile phones are often equipped with a haptic actuator which vibrates to provide a notification for a user upon the arrival of a text message or other similar event. To this end different types of haptic actuators have been developed, among which are piezo-electric actuators, the eccentric rotating mass, ERM, and the linear resonant actuator, LRA.
LRAs are based on an inductive coil (solenoid) coupled to a spring holding a permanent magnet. In operation, the spring and mass system move along a single axis. When a current is passing in one direction through the coil it creates a magnetic field that repels the magnet. When passing the current in the other direction the magnetic field attracts the magnet. Hence, the transfer of energy between the spring and the coil generates oscillations. The system has a mechanical resonance frequency typically in the range of 50-300 Hz. At the resonant frequency, the push-pull drive voltage produces the maximum linear deviation of the sprung mass.
Following Lenz's law, upon oscillation of the mass, a back electromotive force, BEMF, is generated across the actuator that opposes the voltage of the source that created it. When the mass moves through the coil it follows a simple harmonic motion, which causes it to induce a sinusoidal voltage in the solenoid. This BEMF is strongest at the resonance frequency of the system.
The BEMF is proportional in the first order to a drive signal driving the LRA. However, the BEMF amplitude is dependent on the weight and strength of the magnet and the number of turns of the solenoid coil. This means that the BEMF varies across LRAs and across temperature and other electro-mechanical parameters.
The mass of the LRA can be accelerated or decelerated by varying the parameters of a drive signal driving the actuator such as the amplitude and the phase of the signal. In this way, a desired user feedback can be achieved. However, the LRA does not respond immediately to a change in amplitude of the drive signal but instead provides a low-pass filter. The rate of acceleration is proportional to the driving power. Hence, it is possible to improve the response of the LRA by overdriving it for a short period of time.
Current practice uses overdriving to accelerate and retard the LRA's oscillations. The level to which a system can be overdriven will depend on the characteristics of the LRA including its resonant impedance, and response time. However, while overdriving allows the LRA to reach a desired amplitude of oscillation more quickly, it is often difficult to know when to stop the overdriving.
Current approaches require calibrating the system for a specific haptic actuator based on its BEMF response, so that the haptic actuator response to a particular level of driving signal is known. This limits the use of the system to a specific actuator and requires a relatively complex system.